The invention is related to the field of wellbore drilling. More specifically, the invention is related to a method for wellbore drilling in deep ocean water.
In many oil and gas provinces, reservoirs have reached a stage where it is difficult to maintain production rates that can support daily operational and maintenance costs. Infrastructure platform and pipeline systems are in place, but larger fields become more and more dependent on fewer wells producing at lower rates. As a result, much exploration effort is directed at hydrocarbon production from beneath very deep ocean water.
Geological and well-design barriers will eventually prohibit access to ultra-deep water basins using conventional drilling technologies. For example, as water depths increase, so does the number of casing strings needed to overcome problems associated with shallow-water flows, weak formations, lost circulation, underground blowouts, sloughing shale, and high-pressure zones. As deeper formation prospects require the use of more contingency casing strings, conventionally-drilled wellbores eventually may reach a point where progressively smaller wellbore diameters hinder drilling progress or constrain production rates.
One solution to overcome these problems is a drilling system called dual-gradient-drilling, (xe2x80x9cDGDxe2x80x9d). DGD can be used for drilling wells in deep ocean water. In DGD, the effects within the well of a column of returning drilling mud from the sea floor to the surface of the ocean are controlled so as to be substantially the same as if the returning drilling mud column were seawater. This may be accomplished by using a sea floor pump in the mud return system, or by injecting a low-density material near the base of a marine riser.
FIG. 1 shows a diagram of prior art DGD, more specifically for extended-reach or long horizontal well drilling. Typically, a system with DGD circulates drilling fluids down (22) a drill string (2), out a bit (4), up the well annulus (18), through a riser (6) to a floating drilling rig 14 at the surface 32 of a body of seawater, and back to an active mud system (not shown). At the mud line (8) is a blowout preventer (BOP) stack 38 which can close and seal an annular space between the drill string (2) and the riser (6). When the BOP (38) is closed, it stops the returning mud (24) from flowing up the riser (6),. To advance fluid flow up (20) the riser (6), a pump (130) introduces gas (21) or other low density fluid through a boost line (12) to lift the returning mud up the riser (6)). Typically, the amount of gas or low density fluid introduced into the boost line (12) is selected to provide a pressure gradient in the riser (6) equivalent to having the riser (6) filled with sea water. Below the mud line (8), a part of a wellbore is typically cased (24) to prevent the wall of the wellbore from caving in, to prevent movement of fluids from one formation to another, and to improve the efficiency of extracting petroleum if the well is productive. In a reservoir (26), however, the wellbore may be xe2x80x9copen holexe2x80x9d (28), meaning it is uncased. At the wellhead, commonly, a blowout preventer stack (38) and several valves (30) are installed to prevent the escape of pressure either in the annular space between the casing (24) and the drilling string (2) or in open hole during drilling or completion operations.
In designing the circulating system, considerations include annular bottom-hole circulating pressures, hole cleaning requirements, the bottom hole assembly requirements, reservoir fluid influx, fluid regime and economics. In addition, it is important to optimize the bottom-hole pressure, which is affected by many interrelated parameters, for example, types and rates of injection fluids, performance of reservoir fluid inflow and drill string movement. All of these parameters affect bottom hole pressure.
Even though DGD enables drilling in deep water, in long horizontal wells, a significant fraction of the bottom hole pressure results from circulation pressure needed to overcome frictional pressure loss in the return mud circulation system. This pressure loss, and the circulation pressure needed to overcome it, increase as the length of well increases. However, in horizontal wells, the vertical depth of bottom of the well is about the same over the length of the horizontal segment of the well. The fracture pressure therefore does not increase with measured wellbore depth. As a result, the bottom hole pressure eventually will exceed a safe amount, even when using DGD techniques.
In one aspect, the present invention provides a method for drilling deeper than is possible using conventional drilling techniques in deep ocean water by controlling bottom-hole pressure during dual-gradient drilling.
In one embodiment of a method according to the invention, a blowout preventer is closed to stop fluid flow through the blowout preventer, which seals an annular space between a wellbore and a drill string therein, and to divert the fluid flow through a bypass conduit. This is followed by stopping introduction of fluid into the interior of the drill string during the drilling operation. Through the bypass conduit in this embodiment, the lower end of a riser is hydraulically coupled to the wellbore at a point below the preventer. The riser in this embodiment extends from the blowout preventer to a drilling rig at the earth""s surface. Passage of fluid flow is selectively controlled, using a subsea choke operatively coupled to the bypass conduit. The fluid flow is regulated to maintain a substantially constant pressure at a selected depth in the wellbore.
This invention is generally applicable to any DGD system, regardless of the method used to maintain wellbore annulus pressure at the mud line. It is particularly applicable to DGD systems that employ gas or some other diluent to lighten a column of mud in the riser.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.